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stucco applications
The term "stucco" is uniquely American in its use. Stucco by its European origins refers to interior ornamental plastering. The proper term is actually "render or rendering." However for the sake of familiarity, stucco is also known as portland cement plaster. Stucco is a combination of a binder of portland cement, sand aggregate, hydrated lime, chopped fiber and water, which when mixed together makes a plastic mass that is workable. This can be applied by hand or machine to substrate surfaces of solid masonry or concrete; or over expanded metal lath installed over a water-resistive barrier and sheathing that has been attached to structural framing. The traditional application of stucco is a three-coat process consisting of the first coat or "scratch coat," a second coat called the "brown coat" and final or "finish coat." After initial set and cure, stucco in its hardened state is an exceptional cladding material that remains durable for decades. |
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This is the most common application for residential construction.
Typical wood sheathing substrates are Exterior Grade and
Exposure I plywood, Exposure I Oriented Strand Board and
Ligno Cellulosic Fiberboard (Bildrite). These products are
typically gapped a minimum of 1/8" between meeting edges to
minimize the effects of moisture induced expansion, which can
translate into cracks in the finished stucco. The International
Building Code requires that these substrates be covered with
a minimum of two layers of Grade D building paper prior to
the installation of lath. The Minnesota State Building Code has
amended this requirement for the residential code to allow for the
use of other water-resistive barriers over wood based sheathings.
Please see MNLPB publication Stucco in Residential Construction
for specific information as it relates to residential applications. After installation of the water-resistive barrier, 3.4 lb. self furred
galvanized lath is attached to the structural supports by various fastening methods. Stucco is installed in a three-coat process
with a 3/8" thick scratch coat. This is followed with a 3/8" brown coat and finally a 1/8" cementitious finish coat for a final
thickness of 7/8". If an acrylic finish is substituted in lieu of the cementitious finish the combined thickness of the scratch and
brown coats should be 7/8" prior to installing the finish coat. This is especially important if the stucco is included as part of a
fire rated wall assembly.
Fault Tolerant Stucco System: The performance of any wall system is dependent on the individual wall components acting
as a whole. Some materials work better together than others. What was discovered in our computer modeling research is that
commonly used wall assembly materials can yield a corresponding range of hygrothermal behavior from outright failure to
satisfactory performance. One such system that was determined to be fault tolerant and capable of accommodating a range
of observed material properties without hygrothermal failure was a conventionally framed wall as described above, except
that blown-in closed cell polyurethane foam insulation was used in the stud wall cavities in lieu of conventional fiberglass batt
insulation. |
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It has been illustrated that stucco does drain over a variety of water-resistive barriers, however
some experts and the Builders Association of Minnesota (BAM) have concluded that an enhanced
drainage plane is better. This is perhaps rooted in the discussion on water vapor transport. The
philosophy being that it is important to decouple the stucco from the water-resistive barrier. Doing
this, it is reasoned, prevents capillarity continuity (stucco is so tightly bonded with the housewrap
that it [housewrap] loses the desirable properties of water resistance and drainage of incidental
moisture). It also provides a clear and faster drainage path for incidental moisture to evacuate
from in back of the stucco, and also aids in drying the wall. BAM has taken this practice one step
further by requiring that their membership meet these qualifications as a best practice standard to
meet criteria for their self-insurance program.
• How is a Drainage Plane Made? A basic drainage plane is constructed by adding a
drainage mat over the sheathing which has been previously covered with a water-resistive
barrier (WRB). This is followed by a second intermediary layer (generally Tyvek, garden cloth or similar) over the
drainage mat and finally the lath and stucco.
Locally Available Drainage Mats
• Three Dimensional Nylon Matrix: Locally this has been sold under the name brands Home Slicker by Benjamin
Obdyke, Waterway by Stuccoflex, Enkamat by Colbond among others. It typically comes in a roll 39 inches wide and
provides a drainage space of approximately 1/4".
• Corrugated Polystyrene: Masonry Technology Incorporated makes Sure Cavity. This is a perforated, high impact polystyrene with a 3/16" deep corrugated pattern to mitigate drainage. Both of the preceding products are also available with a spun bonded fabric facer to keep the freshly applied stucco out of the drainage plane.
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Steel framing can also be sheathed with wood based sheathing substrates, however more often
than not; steel framing is sheathed with gypsum based sheathing substrates. According to the
International Building Code, sheathing other than wood-based must be covered with a water resistive
barrier consisting of a minimum one layer of ASTM D226 asphalt felt. Considering the
vulnerability of these sheathings however, the Minnesota Lath and Plaster Bureau recommends
a two layer application of water-resistive barrier. After the installation of the water-resistive barrier,
3.4 lb. self furred galvanized lath is attached to the structural supports by corrosion resistant
screws. Stucco is installed in a three-coat process with a 3/8" thick scratch coat. This is followed
with a 3/8" brown coat and finally a 1/8" cementitious finish coat for a final thickness of 7/8". If an
acrylic finish is substituted in lieu of the cementitious finish the combined thickness of the scratch
and brown coats should be 7/8" prior to installing the finish coat. This is especially important if the
stucco is included as part of a fire rated wall assembly. |
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Direct Applied: There is a great affinity between stucco and
concrete block because they are very similar in composition. For
this reason it is not unusual to apply stucco in a direct application
to concrete block. The concrete should be straight, true and
free of any residual dirt or laitance that can compromise the
bond. A direct application to painted block is not recommended.
Masonry joints should be flush to the face of the block and not
struck. Three-coat application entails a 1/4" scratch coat, a 1/4" brown coat and 1/8" of cementitious finish. If an acrylic finish is
substituted in lieu of cementitious, the combined thickness of the
scratch and brown coats should equal 5/8".
Metal Lath over Concrete Block: For most Minnesota
applications, metal lath attachment over concrete block has always been the preferred method regardless of the condition of
the block. This is because the lath provides a redundant measure that ensures the integrity of the attachment of the stucco
to the block and also gauges the final thickness of the application. This is important considering the thermal changes of the
Minnesota environment. Like in direct applications, it is recommended that all of the masonry joints be flush and not struck.
3.4 lb. self furred galvanized lath is then attached to the masonry with corrosion resistant powder actuated fasteners in
accordance with ASTM Standard C 1063. In this system the scratch coat is applied 1/2" thick, followed by a brown coat of 1/4" minimum thickness and a 1/8" cementitious finish coat. If an acrylic finish is substituted in lieu of cementitious, the combined
thickness of the scratch and brown coats should equal 7/8". |
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Direct Applied: Similar to concrete block, cast-in place concrete and stucco have a great affinity
towards one another because they are similar in composition. Problems arise when the concrete
is so dense that it lacks any absorbency which may limit the bond between the concrete and
the stucco. Another issue that may affect the bond is any residual release oils that may remain
on the concrete as a result of forming the walls. A simple power washing may be all that is
necessary to remove any residual oils, however there are also cleaning products available that
may aid in this process. Nonabsorbent surfaces may have to be abraded by sand blasting, wire
brushing, grinding or acid etching to provide a surface acceptable for the application of stucco.
Other options that may mitigate the bond may include applying a dash bond coat of stucco or a
bonding agent prior to installing the scratch coat. Form ties or protrusions resulting from forming
the concrete should be removed or chipped back to provide a flat surface ready for stucco. Three coat
stucco application entails a 1/4" scratch coat, a 1/4" brown coat and 1/8" of cementitious finish.
If an acrylic finish is substituted in lieu of cementitious, the combined thickness of the scratch and
brown coats should equal 5/8". It is recommended that in any direct applied stucco installation, a
trial application be installed and tested to determine the full installations viability. |
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For most Minnesota applications, metal lath attachment over cast-in place concrete has always
been the preferred method regardless to the condition of the block. This is because the lath
provides a redundant measure that ensures the integrity of the attachment of the stucco to the
concrete and also gauges the final thickness of the application. This is important considering the
thermal changes of the Minnesota environment. Like in direct applications, it is recommended
that the wall be free of any residual forming oils and that any form ties or protrusions resulting
from forming the concrete be removed or chipped back to provide a flat surface ready for stucco.
3.4 lb. self furred galvanized lath is then attached to the masonry with corrosion resistant powder
actuated fasteners in accordance with ASTM Standard C 1063. In this system the scratch coat
is applied 1/2" thick, followed by a brown coat of 1/4" minimum thickness and a 1/8" cementitious
finish coat. If an acrylic finish is substituted in lieu of cementitious, the combined thickness of the
scratch and brown coats should equal 7/8". |
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